The main function of mitosis and meiosis is to separate chromosomes into new cells during a process of division. In order for this process to occur, chromosomes have to be pulled to opposite poles of the dividing cell, and centromeres must be broken. Mitotic spindles allow for this process of segregation to occur, pulling sister chromatids to opposite poles, before they are packaged and the cell is divided into 2 new daughter cells .
The mitotic spindle is "the collective term for all spindle fibres that form during meiosis". This spindle is based on a bipolar set of microtubules, with one end embedded within the spindle pole, and the other end pointing away from this pole. In all spindles, the microtubules must be bipolar, and chromatid pairs have to be joined to an opposite spindle pole. This is known as bi-orientation. Any defects in the make up of the mitotic spindle can lead to errors in chromatid segregation .
Microtubules in the mitotic spindle:
Microtubules are the basis of the mitotic spindle. These polymers continuously grow and shrink, and are regulated in the spindle by motor proteins.The spindle contains three classes of microtubules. "Kinetochore microtubules connect the spindle poles to kinetochores on the sister chromatids; in animal cells multiple kinetochore microtubules bundle together to form kinetochore fibers. Interpolar microtubules link the two spindle poles by interdigitating with each other in the midzone of the spindle. Astral micro- tubules extend from the poles away from the spindle and are typically involved in anchoring and positioning the spindle in the cell. Astral microtubules are generally found only in cells that use centrosomes or spindle pole bodies to form the spindle poles." .
The mitotic spindle is assembled during early mitosis. Construction of the bipolar spindle depends on the ability of the spindle to self-organize. Proteins interact with the microtubules to organize them into two antiparallel arrays, which the ends overlapping in the center of the cell. As the spindles grow, some microtubules become attached to kinetochores, connecting chromatids to the poles. Chromatids are connected to the spindle by a process called search and capture .